Atrial Septal Defect, Ostium Primum 

  • Author: Shannon M Rivenes, MD; Chief Editor: Steven R Neish, MD, SM   more...
 
Updated: Aug 11, 2010
 

Background

An ostium primum atrial septal defect (ASD), as seen in the image below, is located in the most anterior and inferior aspect of the atrial septum. It is the simplest form of atrioventricular (AV) canal or AV septal defect. These defects are often associated with trisomy 21.

Gross pathology specimen viewed from the opened leGross pathology specimen viewed from the opened left atrium and left ventricle, demonstrating a partial atrioventricular (AV) canal defect. An ostium primum atrial septal defect (ASD) marked by an asterisk (*) is visualized in the inferior aspect of the interatrial septum. An ostium secundum ASD marked by 2 asterisks (**) is also noted. The mitral valve is cleft and the leaflets are thickened and rolled, suggestive of chronic mitral regurgitation. LA = Left atrium; LV = Left ventricle; MV = Mitral valve.

During fetal development, the rudimentary atrium is divided by the septum primum, except for an anterior and inferior space that is the ostium primum. The ostium primum is sealed by fusion of the superior and inferior endocardial cushions around 5 weeks' gestation. Failure to do so results in an ostium primum ASD. The endocardial cushions also contribute to the complete formation of 2 separate AV valves and the inlet interventricular septum. For this reason, ostium primum ASDs commonly are associated with malformations of these structures.

Ostium primum ASDs may occur in isolation but most commonly present with a cleft in the anterior leaflet of the mitral valve. This is sometimes termed a partial AV canal defect or a partial AV septal defect. In this case, a 5-leaflet AV valve is arranged so that separate right and left components (a tricuspid valve and a mitral valve) are present. The leaflets connect to each other and then adhere to the crest of the interventricular septum. This results in obligatory shunting at the atrial level with no ventricular level shunting. Generally, a commissure is observed between the left superior and inferior bridging leaflets because of abnormal fusion of the left tubercle of the superior and inferior cushions, which results in a cleft in the anterior leaflet of the mitral valve.

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Pathophysiology

Shunting is predominantly left-to-right in the absence of pulmonary vascular disease or significant right ventricular outflow tract obstruction. This results in volume overload of the right atrium and ventricle and pulmonary overcirculation. If the mitral valve cleft causes significant mitral regurgitation, the left side of the heart also becomes volume overloaded. A left ventricle to right atrium shunt can be present, which further overloads both the right and left heart.

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Epidemiology

Frequency

United States

Ostium primum ASDs are most commonly associated with Down syndrome (trisomy 21). The incidence of trisomy 21 is 1 per 800 live births, with an increased prevalence noted in children born to older mothers.

  • The overall risk of congenital heart disease in patients with Down syndrome is 40-50%. Approximately 65% of those affected have some form of AV septal defect.
  • The inherited risk for children of parents who have an AV septal defect is reported as 9-14%.

Mortality/Morbidity

The presence and degree of associated mitral regurgitation and/or left ventricle to right atrium shunting generally determine symptoms.

Those with either no cleft or a cleft with a mild degree of mitral regurgitation are often asymptomatic. Patients typically are referred for evaluation of a heart murmur in childhood and generally survive well into adulthood. However, adults who have not had the condition repaired often become symptomatic from congestive heart failure (CHF) by age 45 years. Rarely, patients are reported to present in the seventh decade of life. Dyspnea on exertion and fatigue are usual adult complaints. Palpitations secondary to atrial fibrillation or flutter also are common.

Those with more severe mitral regurgitation or left ventricle to right atrium shunting often present in the first 2 years of life. Mortality has been reported to be as high as 30% in this subpopulation in the first year of life.

Although relatively rare, pulmonary vascular obstructive disease may occur in patients with long-standing substantial shunts and significant mitral regurgitation.

Children with trisomy 21 are at higher risk than the general population of developing pulmonary vascular obstructive disease at a younger age. Potential reasons for this include chronic upper airway disease, tonsillar and adenoid hypertrophy, and inadequate alveolarization of the terminal bronchioles, leading to a decreased surface area of the vascular bed.

Sex

The male-to-female ratio is 1:1.

Age

Patients with smaller defects and little or no mitral regurgitation may present at any age with a murmur and/or an abnormal ECG. Those with more severe mitral regurgitation typically present with CHF in the first 1-2 years of life.

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Contributor Information and Disclosures
Author

Shannon M Rivenes, MD  Assistant Professor, Department of Pediatrics, Division of Pediatric Cardiology, Texas Children's Hospital and Baylor College of Medicine

Shannon M Rivenes, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Pediatrics, American College of Cardiology, American Heart Association, and American Society of Echocardiography

Disclosure: Nothing to disclose.

Specialty Editor Board

Paul M Seib, MD  Associate Professor of Pediatrics, University of Arkansas for Medical Sciences; Medical Director, Cardiac Catheterization Laboratory, Co-Medical Director, Cardiovascular Intensive Care Unit, Arkansas Children's Hospital

Paul M Seib, MD is a member of the following medical societies: American Academy of Pediatrics, American College of Cardiology, American Heart Association, Arkansas Medical Society, International Society for Heart and Lung Transplantation, and Society for Cardiac Angiography and Interventions

Disclosure: Nothing to disclose.

Mary L Windle, PharmD  Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Pharmacy Editor, eMedicine

Disclosure: Nothing to disclose.

Alvin J Chin, MD  Professor of Pediatrics, University of Pennsylvania School of Medicine; Attending Physician, Cardiology Division, Children's Hospital of Philadelphia

Alvin J Chin, MD, is a member of the following medical societies: American Association for the Advancement of Science, American Heart Association, and Society for Developmental Biology

Disclosure: Nothing to disclose.

Gilbert Z Herzberg, MD  Assistant Professor, Department of Pediatrics, Section of Pediatric Cardiology, New York Medical College; Consulting Staff, Department of Pediatrics, Sound Shore Medical Center

Gilbert Z Herzberg, MD is a member of the following medical societies: American Academy of Pediatrics

Disclosure: Nothing to disclose.

Chief Editor

Steven R Neish, MD, SM  Director of Pediatric Cardiology Fellowship Program, Associate Professor, Department of Pediatrics, Baylor College of Medicine

Steven R Neish, MD, SM is a member of the following medical societies: American Academy of Pediatrics, American College of Cardiology, and American Heart Association

Disclosure: Nothing to disclose.

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ECG from a patient with a partial atrioventricular septal defect. The PR interval is mildly prolonged. Left axis deviation with Q waves in leads I and aVL are present, consistent with a counterclockwise loop in the frontal plane. Right atrial enlargement and an rsR' pattern in the right chest leads also are noted.
Two-dimensional, apical, 4-chamber echocardiogram of a partial atrioventricular (AV) canal defect. The asterisk (*) delineates an area of dropout in the inferior atrial septum at the site of the primum atrial septal defect. The AV valves are separate but aligned at the same horizontal level, consistent with a 2-orifice common AV valve. In systole, the medial leaflets of the right- and left-sided AV valves demonstrate attachments to the crest of the interventricular septum, allowing no ventricular level shunting. RA = Right atrium; LA = Left atrium; RV = Right ventricle; LV = Left ventricle.
Gross pathology specimen viewed from the opened left atrium and left ventricle, demonstrating a partial atrioventricular (AV) canal defect. An ostium primum atrial septal defect (ASD) marked by an asterisk (*) is visualized in the inferior aspect of the interatrial septum. An ostium secundum ASD marked by 2 asterisks (**) is also noted. The mitral valve is cleft and the leaflets are thickened and rolled, suggestive of chronic mitral regurgitation. LA = Left atrium; LV = Left ventricle; MV = Mitral valve.
 
 
 
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